This invention relates to measuring the size of apertures in a material having an array of similarly sized apertures. Specifically, it relates to measuring the size of apertures in shadow masks for cathode-ray tubes.
Shadow masks are used in color display tubes to insure correct color reproduction of objects at the cathodoluminescent screen. One type of shadow mask is made of a thin metal sheet with substantially parallel columns of slots cut into it. The slots are separated within a column by a tie bar and typically have a rectangular shape with rounded ends. Adjoining columns of slots are offset from each other such that the center of the slot in one column is aligned with the center of the tie bar of the adjoining columns. Other shadow masks have other aperture shapes including: complete column apertures similar to the columns of slots except without the tie bars; and circular apertures arranged in columns and rows across the sheet of material.
To insure proper shadow mask quality, a number of techniques have been developed to measure the dimensions of these various apertures. In most methods, the apertures are measured by projecting a beam of light towards one side of the shadow mask and studying the transmitted light on the other side. In the prior art, the laser beam has always had a circular cross-section where it intersects the shadow mask. In some methods, this circular laser beam is made large enough to cover several apertures. Because the beam is so large, only average dimensions for large sections of the apertures can be attained from such systems. The large beam does not provide any detailed information about particular regions of a given aperture.
Another method uses a circular beam that is smaller than the smallest measurable dimension of a given aperture. This small beam is swept over the shadow mask in one direction as the shadow mask moves in a transverse direction. As the laser spot traverses the shadow mask, light sensors on the opposite side of the shadow mask measure the intensity of the transmitted light. In order to calculate a dimension of the aperture, this method divides the shadow mask into square segments with a plurality of square segments assigned to each aperture. To scan an entire aperture, the small beam must traverse the aperture several times while the mask moves in the transverse direction. After scanning a large block of square segments, the dimensions of each aperture can be determined by comparing the relative intensities of each square segment. This system requires both a large amount of memory to track the intensity at each square segment and the ability to uniformly scan the shadow mask.